MYP Design Gears, Pulleys and Cams Notes

Gears Swap Speed for Turning Force

1. A gear is a toothed wheel, and because the teeth mesh rather than rub, the drive between two gears never slips.
2. Spur gears are the plain flat toothed wheels you see in a clock, they sit on parallel shafts and they always turn in opposite directions.
3. Bevel gears have their teeth cut on a cone so they carry drive around a corner, usually through 90 degrees, as in a hand drill or a car differential.
4. A worm gear is a screw meshing with a toothed wheel, and one full turn of the worm moves that wheel on by a single tooth.
5. That gives a huge reduction in one small step, since a worm driving a 30 tooth wheel is already 30 to 1.
6. A worm drive will not usually run backwards, so the output cannot turn the input, which is why guitar tuning pegs and lifting gear use it.
7. Two gears only mesh if their teeth are the same size, measured as the module, so a module 1 gear will never run with a module 1.5 gear however you space the shafts.

Gear Ratio: Just Count the Teeth

1. Gear ratio is the teeth on the driven gear divided by the teeth on the driver gear, and the driver is whichever one the motor or handle turns.
2. A 10 tooth driver turning a 40 tooth driven gear gives 40 divided by 10, a ratio of 4 to 1.
   1. Spin that driver at 200 rpm and the output turns at 200 divided by 4, which is 50 rpm, with roughly four times the turning force.
3. Swap them over, 40 teeth driving 10, and the ratio becomes 1 to 4, so the output spins at 800 rpm with a quarter of the force.
4. A ratio above 1 is a reduction and a ratio below 1 is a step up, and almost every school project needs the first one.
5. A small 6 V hobby motor spinning at 8000 rpm is useless on a wheel, but a 100 to 1 gearbox brings that down to 80 rpm, which is a sensible speed for a moving toy.
6. Stages multiply, so two 5 to 1 pairs in a row give 25 to 1 overall.

Idler Gears Fix Direction, Not Ratio

1. Two meshed spur gears always turn opposite ways, which is a problem when your design needs the output turning the same way as the input.
2. An idler gear is a third gear dropped in between the other two, and it puts the output back the right way round.
3. The idler has no effect at all on the overall ratio, however many teeth it has.
4. Prove it: a 10 tooth driver, a 20 tooth idler and a 40 tooth output give 20 over 10 times 40 over 20, which is 4, exactly what 40 over 10 gives on its own.
5. Idlers also bridge a gap when two shafts sit too far apart for their gears to reach each other.
6. Count the wheels to predict direction, because an odd number of gears in a train means the last one turns the same way as the first.

Belts and Chains Move the Drive Across a Gap

1. A pulley and belt drive uses two wheels linked by a flexible belt, and the ratio comes from the diameters instead of the teeth.
2. A 25 mm driver pulley turning a 100 mm driven pulley gives 100 over 25, a ratio of 4 to 1, the same sum with different numbers.
3. A belt can slip, which costs you accuracy but saves the motor when the output jams, so it doubles as a cheap safety clutch.
4. A toothed timing belt stops the slipping while staying quiet, which is why 3D printers run GT2 belts on the X and Y axes.
5. Twist a plain belt into a figure of eight and the output runs backwards, a direction change that gears need an extra wheel to achieve.
6. A chain and sprocket cannot slip and carries far more force, which is why bikes and motorbikes use chain and a washing machine uses belt.
7. Chain drives need lubricating and a guard, because an unguarded chain will catch fingers, hair and sleeves.

Cams Turn Round Into Up and Down

1. A cam is a shaped plate on a turning shaft, and the follower is the part resting on its edge that gets pushed up and down.
2. One full turn of the cam gives one complete cycle of the follower, so the outline you cut is effectively a timetable for the movement.
3. A pear cam lifts the follower for part of the turn and then holds it still for the rest, and that pause is called dwell.
4. A snail cam raises the follower slowly and then drops it suddenly off a step, and it only works turning one way.
5. An eccentric cam is just a circle mounted off centre, giving a smooth even rise and fall with no sudden jolt.
6. The rise is how far the follower travels, and it equals the largest radius of the cam minus the smallest.
7. Followers come in three kinds: knife edge is precise but wears fast, flat spreads the load, and roller has the least friction but needs the most room.

Start From the Motion, Then Pick the Mechanism

1. There are four kinds of motion to name: rotary turning, linear straight one way, reciprocating straight back and forth, and oscillating swinging about a pivot.
2. For rotary to reciprocating use a cam and follower, or a crank and slider like the piston in a petrol engine.
3. For rotary to linear use a rack and pinion, a round gear running along a toothed bar, which is what moves a car steering rack and a pillar drill quill.
4. A leadscrew does the same job much more slowly, running a nut along a threaded rod, and it is the right choice when you want strength and fine control rather than speed.
5. Run a crank and slider the other way and reciprocating motion becomes rotary, which is how pistons spin a crankshaft.
6. Write down the motion your user needs first, then choose from this list, rather than picking a mechanism you like the look of and bending the product around it.
7. In most projects a microcontroller decides when the motor runs, and the code for that sits in the programming articles rather than here.